How to Create A Cmake Variable?

To create a CMake variable, you simply need to use the set() command followed by the name of the variable and its value. For example, to create a variable named "MY_VARIABLE" with a value of 5, you would write:

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set(MY_VARIABLE 5)

You can also create variables based on the values of other variables or CMake properties. Additionally, variables in CMake can have different scopes, such as global, directory, or function scopes. It is important to be aware of variable scopes when creating and using variables in CMake.

How to access a CMake variable in a script?

To access a CMake variable in a script, you can use the configure_file command in your CMakeLists.txt file to generate a header file with the variable value. Then, you can include this header file in your script to access the variable.

Here's an example of how you can do this:

1. In your CMakeLists.txt file, define a variable and use configure_file to generate a header file with the variable value:

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# Define a variable
set(MY_VARIABLE "Hello, World")

# Generate a header file with the variable value
configure_file(config.h.in config.h)

1. Create a config.h.in file in the same directory as your CMakeLists.txt file with the following content:

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#define MY_VARIABLE "${MY_VARIABLE}"

1. In your script, include the generated header file to access the variable value:

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#include "config.h"
#include <iostream>

int main() {
    std::cout << MY_VARIABLE << std::endl;
    return 0;
}

1. Build your project using CMake:

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mkdir build
cd build
cmake ..
make

1. Run your script to see the output:

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./my_script

This way, you can easily access CMake variables in your script by generating a header file with the variable value.

What is the difference between a CMake variable and a CMake property?

In CMake, variables are used to store and manipulate values within the CMake script itself, while properties are used to set attributes or options for targets, source files, and other CMake entities.

Variables in CMake are typically used for storing values such as directory paths, compiler flags, build settings, or custom parameters that are used across the script. These variables can be set, modified, and referenced within the CMake script using the set() command.

On the other hand, properties are specific attributes or options associated with CMake entities such as targets, source files, or directories. Properties are set using the set_property() command and are used to customize the behavior or settings of these entities. For example, properties can be used to set compile options for specific source files, define custom commands for targets, or specify installation rules for certain files.

In summary, variables are used for general-purpose value storage and manipulation within the CMake script, while properties are used to set specific attributes or options for CMake entities such as targets, source files, and directories.

What is the difference between a CMake variable and an environment variable?

CMake variables are internal variables used by the CMake build system to control various aspects of the build process, such as compiler flags, linker flags, options, and settings specific to the CMake build system itself. CMake variables are defined and manipulated within CMake scripts and are used to customize the build process.

Environment variables, on the other hand, are system-wide variables that are set outside of the build system and are used by various programs and processes to determine settings and behavior. Environment variables are set at the system level and can be accessed by any program or process running on the system. They are not specific to CMake or any particular build system.

In summary, CMake variables are used internally by CMake to control the build process, while environment variables are system-wide variables used by various programs and processes to determine settings and behavior.

What is the use of the CACHE option in CMake variable?

The CACHE option in CMake variable is used to create a cache entry for a variable. This allows the variable to be set by the user when running CMake configuration and store the value in the CMake cache file. This value can then be used in subsequent CMake runs without having to set it again. This option is useful for setting configuration options or user-defined variables that need to persist between runs of CMake.